Ultrasonographs that obtain in real-time a blood flow signal in a test subject with use of an ultrasonic beam emitted from an ultrasonic probe conventionally have been used. With such ultrasonographs, depending on the frequency of the detected signal, there are cases in which aliasing occurs in an obtained Doppler image, and when such Doppler image aliasing occurs, it is necessary for an operator to adjust the pulse repetition frequency according to the blood flow velocity.
In view of this, in order to not force the operator to perform such a troublesome operation, there is known technology in which by detecting a signal existence region corresponding to a detectable frequency range, the repetition frequency (PRF) of ultrasonic beam transmission/reception and a baseline position (0 Hz position) are adjusted automatically so that the Doppler image does not cause aliasing to occur (e.g., see Patent Document 1).
In this technology disclosed in Patent Document 1, first an operation for classifying a Doppler spectrum obtained by FFT into a “value 0” portion and a “value 1” portion with use of a threshold value is performed on a certain range of a Doppler image. Next, a signal existence ratio, that is to say, a percentage occupied by the signal existence region, is obtained, and the PRF is set such that the signal existence ratio is approximately 0.6 to 0.8. Then, a signal existence region is obtained with respect to a Doppler spectrum obtained with use of the set PRF by performing a similar operation, and the baseline position is moved such that the center of the region in which the signal does not exist moves towards the edges of the screen.
On the other hand, technology also has been proposed in which, in the case in which the intensity of the blood flow signal changes, such as the case in which an ultrasonic contrast agent is used in blood flow measurement, in order to optimize the intensity of the obtained blood flow signal, the level of a signal retrieved from a Doppler signal detection unit is detected, and the gain of the signal is automatically adjusted (e.g., see Patent Document 2).
In this technology disclosed in Patent Document 2, an appropriate gain value that is in accordance with the signal level of the Doppler blood flow signal is determined with use of a table stored in a storing means such as a ROM in advance, thus automatically optimizing the gain, which is the luminance signal of the obtained Doppler image.    Patent Document 1: JP H08-308843A    Patent Document 2: JP 1108-173426A